Temperature control switch



Oct. 16, 1951 D. l. BOHN 2,571,822

TEMPERATURE CONTROL SWITCH Filed April 6, 1948 4 Sheets-Sheet l INIII? VI II lll Oct. 16, 1951 D. l. Bol-1N 2,571,822

TEMPERATURE CONTROL SWITCH Filed April 6, 1948 4 Sheets-Sheet 2 www,

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/ITTORNE Y6' Oct. 16, 1951 D. 1. BOHN TEMPERATURE CONTROL SWITCH 4 Sheets-Sheet 5 Filed April 6, 1948 INI/EN'IOR. DNALD f. QOH/V TTORNE YS Oct. 16, 1951 Filed April 6, 1948 11n/M aycLE 1J Wag/M off D. l. BOHN TEMPERATURE CONTROL SWITCH filza/naam? OPEBATIN@ 4 Sheets-Sheet 4 cm3 V5 .DEGPEES FAHRENHEIT INVENTOR.

Patented Oct. 16, 1951 UNITED STATES PATENT OFFICE TEMPERATURE CONTROL SWITCH Donald I. Bohn, Pittsburgh, Pa.

Application April 6, 1948, Serial No. 19,241

(ci. 20o-136.3)

Claims.

My invention relates to a novel temperature control system, and more particularly to a novel apparatus for a method o`f modulating a heat supply in accordance with the difference ibetween anindicated desired temperature and the actual temperature, achieved by utiliizng a modied servo principle for temperature control.

vIn systems heretofore in use, one or more thermal units have been employed which turn on a source of heat when the temperature drops below a predetermined value, the heat supply being kept on until the temperature has been reached. Such systems, however, have been found to be unsatisfactory because of the heat lag which causes the temperature to rise considerably above the desired value after it has been cut off and which permits the temperature to fall to a relatively low value before the heat supply can stop the trend. Temperature uctuations of as much as 6 result from the present temperature control systems.

In accordance with my invention, the heat supply may be considered asl continuously applied cyclically at a predetermined frequency. The cyclical application is,` however, Variably modulated in accordance with the temperature difference between the indicated desired temperature and the existing temperature, the modulation consisting in cutting the heat supply on or on by variable amounts in accordance with this temperaturev difference.

lll

Thus, assuming by way of illustration a heat cycle of four minutes, and the room temperature below the desired temperature, in accordance with myn'vention the temperature control functions to supply continuous heat until the room temperature approaches to within a predetermined value of the desired temperature, which again by way of illustration may be within 2 yof the desired temperature'. From this point on, the control automatically functions to modulate the four minute cycle; that is, I cut 01T the heat in each such four minute cycle by a varying amount in accordance with the above mentioned temperature difference, the off time period being smaller when the temperature difference is great and the oilv period being greater as the temperature difference becomes smaller; or looking at this from a heat supply point of view in each four minute cycle the on period is longer when the above indicated temperature difference is great and the on period is relatively less when the indicated tem- I perature difference is smaller.

At some points in such a modulated system, the heat losses 'equa-1 the heatsupply and a state of 2 equilibrium is achieved in this dynamic system and this condition is maintained so long as this balance maintains. While this state of equilibrium obtains, the on and ofi` time of the heat supply remains constant.

With this application of the servo principle, I have in practice found that I can secure such a state of stability or balance at any point within a range of 2c and I can reduce this temperature range by either decreasing the period of the cycle, i. e., increasing the frequency of the cycle, or by applying auxiliary thermal control to my thermal control mechanism.

Accordingly an object of my invention is to provide a novel cylically operable thermal control.

A further object of my invention is to provide a heat control system having a predetermined on and off period which is modulated in accordance with the difference between a desired and the existing temperature.

Still a further object of my invention is to provide a novel heat control system in which continuous control is secured of the heat supply until a state of equilibrium between heat losses and heat supply is obtained.

Still another object of my invention is to provide a control in which the heat supply is continuous at a predetermined frequency in which the relative on to oi period is variably controlled in accordance with predetermined desired temperature conditions.

There are other objects of my invention which together with the foregoing will appear in the detailed description of the drawings in which Figure l is a schematic illustration of the complete thermal control and regulator.

Figure 2 shows the details of the thermal control showing continuous heat application when the room temperature is more than a predetermined value below the desired temperature.

Figure 3 shows the same thermal control functioning to modulate the heat supply when the room temperature is near the desired value.

Figure 4 shows a set of curves illustrating the principle on which the device operates.

Referring to Figure l, a vapor lled bellows I of well known construction is responsive to the temperature of the space being controlled. A rising temperature causes expansion and a drop in temperature causes a contraction of the bellows. On expansion the bellows exerts a pressure through engaging surface 2 against a lever arm 3 and-thus tends to move the lever 3 to the left.

Lever arm 3 is pivotally supported against a fixed pivot il about which it oscillates through a small angl-e. As mentioned, the bellows tends to cause lever arm 3 to rotate countercloclrwise about pivot Il. This action, however, is restrained by the upward force of the iloating ulcrum 5. Fulcrum Sis secured to one of the links, the other end of which carries the bottom end of the spring 6, the top end of which is fastened to non-rotatable nut I adjustable axially along the screw shown by thumb-screw 8. The vertical position of nut 'I is indicated by pointer 9 adjacent to -a temperature calibrated scale I0.

It will be observed that when the bellows has4 expanded in response to the room temperature, it exerts a force on the lever 3 tending to move it counterclecliwise about pivot l. Simultaneously the tension spring 5 exerts a force tending toV rotate the lever 3 clockwise about pivot il; At.

some predetermined temperature these forces are in equilibrium and there is no resultant turning torque applied to lever 3.

The design of the parts is such that lever arm 3 rotates about pivot through its entire motion of a small angle with a change in room temperature of about 2 F. This operating motion is limited by stops i! so that the above mentioned 2 F. of roomtemperature change will cause lever arm 3 to oscillate at full range between the two stops 'l l. The actual room temperature at which this motion will occur is adjustable and predetermined by means of thumb-screw 8' which by varying the tension on spring 3 determines the temperature at which a balance of the spring and bellows is secured. This mean temperature value is indicated by pointer 9 against scale i9.

Mounted on and rigidly fixed t lever arm S' is a small piece' of Bakelitesheet I2 which carries a'pivoted lever arm I3 operating on pivot pin I4, iastened'to Bakelite sheetV I2. This lever arm I3 also made of Bakel-ite carries a contact arm I made of a spring material, such as phosphor bronze; land carrying a movable contact I6, as shown. This movable contact I6 will at times, depending upon the difference between thev desired temperature indicated at 9 and the actual temperature measured by the bellows I, engage lower contact Il' which is fastened to Bakelite sheet I2. Connections from both of these con` tacts to terminals IS are made with flexible leads.

Mounted adjacent to theupper end of pivoted lever arm I3, in about the position shown, is cam I9 driven by a gear 20 meshing with pinion IIS mounted on a shaft of a small synchronous clock motor 4I.. Cam I9 is designed as an eccentric with a flattened portion on its minimum radius side, as shown.

The lever arm I3 is pivoted at pin I4 so that when the flattened portion of cam i9 is Opposite thelever arm I3, the lever arm I3 has been moved toitsY extreme counterclockwise position and the contact IzS on the end of the contact arm i5, which is. inan extension ofV lever armY i3, will be moved tcengage the fixed contact Il. As the gear 20 is now rotated by its motor, in turn rotating the cam I9, the cammed or enlarged porr tion of the cam I9 moves to engagement with the arm I3, and rocks the arm I3 about its pivot I I in a clockwise direction to effect a, disengagement between the contacts IE and II. The position ofthe lever I3 when the contacts I6 and Il are in. engagement is shown by the dash lines in Figure 2. The position of the lever I3 when the camhas rotated so that the enlarged portion engages lever YIlis shown by the full line in Figure 2, at

:l to suit various conditions.

4 which time contacts l5 and II are in disengagement.

In one form of the invention, synchronous clock motor 4I operates continuously and rotates cam I9 at a speed of one revolution in four minutes. This speed of rotation is a compromise between selecting a rate of rotation which is much faster to provide more accurate temperature control than is necessary and which would cause an undesirably large number of operations in a, day, and too slow a period of rotation which wou-ld actually permit an appreciable temperature variation to exist during the period of one rotation. This speed or" cam rotation, therefore, may be selected It has been found by trial that one revolution in four minutes is quite satisfactory for a normal house, both from a standpoint of providing perfect control and yet ...not causing an excessively high number of gas valve operations in a certain period of time.

Before following through the entirecircuit system, it is rst necessary to have a comprehen- Y sion of the operation of this thermostat without With the room temperature at 60 F., lever arm y 3 will be in its eXtreme clockwise position and the relationship of the various parts when in this position is. such that movable contact It touches lower contact Ill all of the time, as under this condition the upper end of pivoted lever arm I3 will be far enough to the right of cam I9 so that cam le does not touch the upper end of pivoted.

lever arm I3 at any point of its entire revolution.

This condition is illustrated inrfullV line in Figure 2 and the dotted line in Figure 3. As will be clear from the dashed position of Figure 3, the member I2 is in its extreme right position due to the contraction of the bellows and accordingly with cam I9 Vtowards lever I3, it will Still not engage the` leven Accordingly the contacts I5 and I6 remain engaged as shown in the dotted position. In Figure 2 member I2 and lever I3 are shown in full with the member I2 still in its extreme right position and of course the contacts remain closed.

The gas is now turned on continuously, regardless of the operation of cam I9, until the room temperature reaches and slightly exceeds 69 F. When this temperaturev condition 'is reached,jBakelite member I2 is moved to the left by the bellows. and lever arm 3 will leave the right-hand stop II. The movement of member I2 to theleft will bringthe upper end of pivoted lever arm I3 closer to cam I9. Y

A very slight movement in this direction is suicient so that the longr radius portionof cam I9 will touch the upper part of pivoted, lever arm I3 fora short periodv of time each revolution.

forcing it-to rotate clockwise through a small angle aboutpivot pin I4 andcausing movable' contact I6 to separate from lower contact I'I.

This condition is shown in thedottedV linesA of@ Figure 2 and the full lines of Figure 3. In Fig-- ure 2 with the cam I9 as shown, lever I3 has followed thel camvand the contacts are closed. In

Figure 3- with the cam rotated 180, leverl I3has been rocked about its pivot to disengage contact I6 from contact I1.

Thus it will now be evident that Figure 2 shows the condition which obtains when the large radius of the cam is away from lever I3 both for conditions when the room temperature is below the desired value shown in the full lines with the parts in their extreme right position, and when the room temperature is near normal shown 1n dotted lines with the parts moved to the left.

Figure 3 shows the conditions which obtain when the large radius of the cam is toward lever i3 both for conditions when the room temperature is below the desired value shown in the dotted lines with the parts in their extreme right position, and when the room temperature is near normal shown in the full lines with the parts moved to the right.

It will be observed from the above that until the room reaches 69 F., the gas is on all of the time; that is, it is on for 240 seconds of the 240 seconds representing one revolution of cam i9.

As soon as the temperature slightly exceeds 69 F., the above mentioned action cuts oil the gas for a few seconds during each cam revolution, and this period of cut-off is gradually increased as lever arm 3 is turned through a small counterclockwise angle about pivot 4 in response to the action of bellows l and the rising room temperature. Y n

Consideration `of this entire operation will show that at some position oi lever arm 3 the seconds per four minute cycle during which the gas is turned on will just provide suicient heat to balance the heat losses and is maintained at this temperature. At this point of equilibrium, the lever arm 3 will stop its counterclockwise motion.

For example, if the weather conditions are such that a temperature of 70 F. will result with the gas on 120 seconds out of every 240 seconds, then this equilibrium will be reached with the room temperature at 70 F. and lever arm 3 in its midposition, this position providing such 120 second gas operation per 240 second cycle.

-It may also be that the weather conditions caused a rate of rise such that the temperature is stabilized at '70.5 F. with the gas on sixty seconds out of a 240 second cycle, in which ease this will be the point at which equilibrium will be established.

From this description it will be seen that equilibrium will always be established within the 2 F. range representing the regulation of the device. Hereafter additional detail will be discussed for reducing this range.

Referring now to Figure 4, I have here shown a curve in which the time of heat application per four minute cycle is plotted against temperature. It will be noted that for a setting of 70 F., curve 80 is substantially a straight line from 69 to 71 indicating a linear characteristic. Curve 8| which is the rst derivative of curve 80 showing the slope of the curve, illustrates the now obvious fact that a state of equilibrium is achievable between a and b at any temperature between 69 and 71 for this particular setting. v

Curve 82 shows the curve obtained when the control has been made more sensitive. Here a straight line curve is obtained between 69 and 70.5, .the equilibrium as shown by the rst derivative curve 83 being between c and d.

Referring again to Figure l, the left-hand assembly on this figure indicates a control panel mounted in the basement, entitled Basement Panel. Atv the top of this panel, terminals 22 receive the main power source assumed to be volts, 60 cycles, single phase. Connected across these loads is the primary of a small transformer 23 which provides a 24 volt output for certain parts of the scheme on which it would be undesirable to impose 115 volts.

This panel contains two relays, one a series relay 24 and the other shunt relay 25. Series relay 24 is provided with a coil which will close its relay contacts when normal current as required by the 115 volt fan motor is flowing. Shunt relay 25 will7 close its contacts when 24 volts, 60 cycles, is applied to its coil.

Assume that the equipment is just being placed in operation and that contacts I6 and Il of the room thermostat are not touching. It will be observed that the coil of relay 25 will be de-energized and therefore its contacts will be open. Similarly, relay 24 will be de-energized since there is no fan current passing through its series coil.

As soon as thermostat contacts I6 and I1 engage, as described previously, relay 25 will be energized, and both of its contacts will be closed. Contact 25a of relay 25 will therefore provide a path for current to the fan through the series coil of relay 24. This will initially start and run the fan motor and will also energize relay 24 because the motor current will be flowing through the series coil of relay 24.

As soon as relay 24 has closed, which occurs instantly after the closing of contact 25a, its,

contact 24a, which is in parallel with the contact 25a of relay 25, will provide a parallel path for fan motor current, thus sealing in relay 24.

So far the operation of relay 25 has been described with reference to its function of starting the fan motor. Closing of contact 25h of relay 25 and contact 24D of relay 24 will complete an energizing circuit for the solenoid gas valve which will be energized and the gas will be turned on in the furnace.

The gas will remain turned on until in accordance with the previous description of room thermostat operations contacts I6 and I1 separate. Therafter relay 25 will be de-energized opening both of its contacts. This will shut off the gas to` the furnace, but it should be observed that relay 24 will still remain closed and the fan motor will continue to operate. y

At this point it should be explained that the basic principle of this entire scheme demands that the circulating fan operate continuous unless the temperature outside rises to such a point' that no heat is required from the furnace; and after this condition has existed for a short period of time, such as thirty or forty minutes, the fan should be automatically shut down, as there is no need for its continued operation.

This feature is taken car of by a pivoted arm 26 pivoted at 28 and carrying a smal1 counterweight 21, which is fastened rigidly to the arma.- ture 25e of relay 25. When relay 25 is energized, arm 26 is raised away from engagement with threaded sha-ft 29. When relay 25 is de-energized, however, arm 26 rests against the thread of threaded shaft 29 as shown. Threaded shaft 29 is rotated slowly (in this case at x/3 R. P. M.) by means of a small synchronous clock motor 30 which is connected electrically, as shown, to operate only when the fan motor is operating.

It will be seen that this arrangement of devices is such that during the de-energized period of 7 relay 25, arm l2li 'which co'ritacts the thread of threaded fshaft 3279 -isf'c'aused to move 'to the left by vir-'tue of the rotation of `threaded shaft 219, and or `'course it will progress 4:one thread every three I'minutes If `:the room ltlfieilmostat trhas not vcalled v:for :any heat -for a long period "of time, 'such vas "thirty or forty minutes, arm A2-"6 will'haveprogressed far enough to the lett "on threaded vv'shaft 2"9 sot-hat it will contact micro-switch '31 and close its contants. lThese contacts are v'in parallel with `the series c'oil of r'elay -2-4 so tha-'t the `rclosing of micro-sWi-tchlSvl Will cause reIayQ-d to 1e-energize, thus shutting down -the fan niot'or `and incidentally stopping the rotation-'of threaded shaft 29.

This condition "will now exist until, due to a drop in outside temperature for -'a `manual 'readjustment of the -i'oom thermostat, heat is again called for by'contacts fr@ and :ITI being eausedt'o touch. When tliis happens, relay-25 lis energized, the .fan 4motor iis started, `gas is turned-on, `arirn 2-6 is lifted from contact with 'threaded Ashaft 29, micro-switch 3| is opened, and normal 'operating conditions obtain.

This control scheme provided -by the -basement panel, therefore, takes care of the following functions:

.1. -I'f the fanhas not been operating, it will be caused to start at Vthe rst demandfor heat from the room thermostat.

2. The fan will continue to operate-for aperiod of thirty to forty minutes if `'during `this 'period noheat is called for by their-oom thermostat.

`3. At the expiration of this period, the fan Will'be caused to shut downan'd will remain down until heat is again called for.

4. A safety feature Yis provided in that the gas cannot-be turned on unless the Vfan is in operation, seriesre'lay 2li insuring'this.

As pointed out hereinbefore, Athe thermostat has a regulation -oi about 2 This means that if it were adjusted at the iigure above rnentioned, 70 F'., it would hold the exact temperature of 70 F. only when the temperature outside 'was such that a heat balance was established with the gas on half the time, or 120 'seconds 'out of every '240 second cycle.

VIf with ythe thermostat so adjusted the outside temperature should ri'se'to, let us say-68 F., then heat balance 'will `be established with lever arm '3 in 'nearly'its left-hand position. Because of `this 2 F. regulation, the temperature in fthe house will be lheld-at about '71 F. as equilibrium is established at this gure. Conversely, if with the same thermostat setting, the outside temperattire is very low and substantially 'fulltime gas operation is required, then `equilibrium will be established with lever arm 3 at about its lmaximum clockwise position necessary to Vprovide full 'or nearly full time gas operation. This position of equilibrium will be Vestablished at about 69 lFrom the above it will now be clear 'that "in accordance with the disclosed novel temperature control, substantially no iluctuationin lroom temperature is possible. Once the 'point 4of equilibrium has been reached at which the losses equal the heat supply, the percentage onperiod of the heatwil maintain 'that precise 'temperature condition exe'ept 'for what iniinitesimal var-iation may occur within the four minute cycle period between the on period of the heat and the off period of the heat. l

As the outside temperature changes slightly or for any other` reason, the rate of heat loss varies and the control here shown will respond 8 instantaneously t0 change Ithe percentage time of the heat supply in each =cycle -fto relate the new heat supply Vto the new fheat loss.

'It `Wil-1 be 'obs'erved that a maximum "variation of 2 F. cannot Lexist `over -a Ashort time ip'eriod, such `as a dayo'r so, Aso ltlfiatthis 2 diierential is Aactually of no great -momen'tlas the thermostat *is probably manually adjusted many times between Aa very mild 'Weather period andan extremely cold lweatherperiod.

While this condition, therefore, would not make :worth while any very vexpensive for cornplicated Icorrectivescheine to eliminate `it,-I have proposed v'a construction which with a negligible cost and substantially vno complieationsreduce's the temperature range.

lResistor AI2-'I is 'of Suchla v'alue1thatif-24 volts were applied to it continuously, its ehea'tacting on vbellows I to Which it is -adjacent Vwould 'cause an expansion of the bellows equivalent toflet its say, 1;5 5F. atljustment l'of thumb-screw 33. Of course, if it were 'connected in vthis manner it would be or `no help -inerely providing Sa -`xed change in vcalibration -of the thermostat.

vIn order to largely correct Afor this `2" SF. regulation above mentioned, this resistor -2I is Iconnected tto -contacts le and 4I l. It should bef-mentioned l'that the 'Wattage required inlresis't'or -2I is -so small 'and its resistance therefore -so Shigh that when contacts I6 and I1 are nottouching and it is operating in `series with th'e `coil of relay 25, Vit receives substantially vfull voltage. Conversely, the Vvcurrent which resistor 2li permits to flow through the coil of relay 25 when contacts IB and Il are lopen is only a 'very small percentage 'of the current-required Lto close relay 25. Its Vconnection-in the manner shown, therefore, does not in the slightest degree aiect lthe operation of `relay 125 as i-des'crib'ed previously.

With 'this fconneotion it will be observed `that resistor 2i is energized during the -p'er-iod that the gas is not turned on the lfurn'a'ce. It :should also "be 'mentioned that the Vnte'rrnitte'nt applications 'of voltage togresistor 122i tvillprovide asibstantially stead-y biasing "heat to ya'ifeot bellows "l dueto the thermal storage 'involved in the various elements. Due :to this thermal rstorage `the heater 4effect lis that based `on lthe average inplit over a long period of time. -l-t therefore fiin'ctions at a fcorre'otive fdevioe which pro'vides an artificial ambient on the thermostat ba'sed on the general percent ofi loperiation of the thermostat. Y

lf, as previously assumed, thevalue 'foi resistor 2| is selected iso 'that with `voltage applied to it continuousiy the calibration of the ithermostat would 'be :affected 1.5c F., -then'the actual connection employed ft'o :provide intermittent energy to resistor :2l Aivi-l1 reduce ythe 2 regulation inherent `in the -mechanics I'of 'the scheme to .the difference between these `tw`o ifigures, lor .5 F.

ABy slightly `l'ow'e'iling the fresistance vof res'ietr 2l ffurther, this regulation :could 'be theoretically reduced 'to `0 Fnand `it may work :perfectly under this condition, but there is afchance that hunting and overshooti'ng lwill result if this is ldone. In any event, resistor 2li as fdescribed lprovides a simple :and inexpensive f'soh'erne -t'o substantially eliminate the 12 SF. regulation and this may lb'e consdereda minor advantage.

a modified form, tI may :reduce lthe cycle time to three Sminute, two zminute, or even shorter periods for the Arotatiorrof the cam 19. v"Reduction of the 'cycle time increasesthe rapidity of corrective effort required by changing tempera- `9 ture conditions and accordingly would decrease the fractional degree variations occurring during a cycle.

In the above I have described my invention applied to a hot air circulating system. It will be obvious, however, that it is equally applicable to other forms of heat, such as hot water systems.

In the latter it is presently the practice to maintain water around the furnace jacket at a predetermined temperature by hydrostatic control. A water pump operating in response to a temperature responsive element in the room whose temperature is to be controlled, circulates the water from the jacket when there is a demand for heat. As pointed out above, such a system with its inherent heat lag, creates a wide fluctuation in temperature.

It has the further disadvantage in many systems in which domestic heat for the water t0 be consumed is supplied by coils in the water jacket that when there is a radiator heat demand, the water jacket temperature drops rapidly, thus momentarily removing the domesticY hot Water supply.

In accordance with my invention, I avoid these disadvantages by intermittently operating the pump circulator to send the hot water to the radiators intermittently under the thermal control described above. Alternatively, I may do away with the hydrostatic control and instead apply intermittent heat to the water jacket of the furnace in accordance with my novel control.

While I have shown the heat supply as being turned on and oil, it will also be apparent that I may Ymodulate the heat supply by varying the degree of gas valve opening between an upper and lower limit under operation of my control device. AThe basic concept of my invention may take many forms and I do not want to be limited except as set forth in the appended claims.

, I claim:

1. In a temperature control device for controlling the supply of heat to a room, a vapor-filled bellow type temperature control device, a lever, a fixed pivotal support for said lever, said bellows engaging said lever for rocking it about said pivotal support in accordance with the expansion of said bellows in response to temperature rise in the room being controlled, a spring biased floating fulcrum for said lever for biasing said lever against the action of said bellows, an adjusting member for adjusting the bias of said spring, a temperature calibrated scale adjacent said adjusting member, said adjusting member indicating on said scale the adjustment of said spring bias, a pair of stops on opposite sides of said lever for limiting the angle of movement of said lever to a movement corresponding to a two degree temperature response of said temperature control device, a member rigidly secured to and movable with said lever and carrying a rst movable contact, an arm carried by said member and having a second movable contact secured thereto for engaging and disengaging said first movable contact, a cam having a surface engaged by said arm, a synchronous clock motor connected to and driving said cam, the connection from said motor to said cam being constructed to drive said cam at a speed of one-quarter R. P. M., said cam during one portion of its rotation being in engagement with said arm and moving said second movable contact toward disengagement with said rst contact and said rst contact being moved toward disengagement with said second contact by expansion of said temperature responsive 10 device in accordance with the setting of said biasing spring to effect engagement between said contacts for variable periods of time during each complete revolution of said cam.

2. In a temperature control device for controlling the supply of heat to a room, a temperature responsive device, a lever, a fixed pivotal support for said lever, said temperature responsive device engaging said lever for rocking it about said pivotal support in accordance with the operation of said temperature responsive device in response to temperature rise in the room being controlled, a spring biased noating fulcrum for said lever for biasing said lever against the action of said temperature responsive device, an adjusting member for adjusting the bias of said spring, a temperature calibrated scale adjacent said adjusting member, said adjusting member indicating on said scale the adjustment of said spring bias, a pair of stops on opposite sides of said lever for limiting the angle of movement of said lever to a movement corresponding to the temperature range within which the room is to be maintained, a member rigidly secured to and movable with said lever and carrying a first movable contact, an arm carried by said member and having a second movable contact secured thereto for engaging and disengaging said rst movable contact, a cam having a surface engaged by said arm, a motor connected to and driving said cam, the connection from said motor to said cam being constructed to drive said cam at a speed of one-quarter R. P. M., said cam during one portion of its rotation being in engagement with said arm and moving said second movable contact toward disengagement with said rst contact and said rst contact being moved toward disengagement with said second contact by said temperature responsive device in response to temperature rise in accordance with the setting of said biasing spring to eiect engagement between said contacts for variable periods of time during each complete revolution of said cam.

3. In a temperature control device for controlling the supply of heat to a room, a temperature responsive device, a lever, a pivotal support for said lever, said temperature responsive device engaging said lever for rocking it about said pivotal support in accordance with the expansion of said temperature responsive device in response to temperature rise in the room being controlled, a spring for biasing said lever against the action of said temperature responsive device, an adjusting member for adjusting the bias of said spring, a temperature calibrated scale adjacent said adjusting member, said adjusting member indicating on said scale the adjustment of said spring bias, a member rigidly secured to and movable with said lever and carrying a rst movable contact, an arm carried by said member and having a second movable contact secured thereto for engaging and disengaging said rst movable contact, a cam having a surface engaged by said arm, a motor connected to and driving said cam, the connection from said motor to said cam being constructed to drive said cam at a speed of the order of one-quarter R. P. M., said cam during one portion of its rotation being in engagement with said arm and moving said second movable contact toward disengagement with said rst contact and said rst contact being moved toward disengagement with said second Contact by expansion of said temperature responsive device in accordance with the setting of said biasing spring to effect engagement between said contacts for variable periods of. time duringf each complete revolution of said cam.`

4. In a temperature control device for control?- ling the supply of heat toa room, a temperature responsive device, a lever',y apivotal support 'for said lever, said temperature responsive device engaging said* lever for rockingV it about said pivotal support in accordance with the expansion ofsa'i'd'A temperature responsive device in response to temperature rise. in the room being` controlled, a spring for biasing said lever against the action ofsaid' bellows, a member rigidly secured'to and movable with said lever and carrying a rst movable contact', an arm carried by said member and:` havingu a second movable contact secured thereto for engaging and disengaging said rst movable contact; aV cam having a surface; engaged by' said arm, ad motor connected. to. and. driving saidcam, the connection from'said motor to said cam' being constructed todrive said cam at a speedof the order ofv one-quarter' R; P: M., said cam during onel portion of its rotation being in engagement with said armand moving said sec'- ond movable contact tovvarddisengagement with said first contact and saidV rst Contact' being moved towardy disengagement with saidy second conta-ct by expansion of said-temperatureY responsive device in accordance with thesetting ofisaid biasing* spring-to eiTect engagement between said contacts for variableperiods oi time duringY each complete revolution off said cam'.

52 Ina temperature control devicef'orcontrolling the supplyof heat toa room, a temperatureA responsivev device, a 1 evera xed pivotal support for saidt lever, said' temperature responsive device engaging; said lever*v for rocking it about. said pivotalsupport in accordance with the expansion of said' temperatureA responsive-,device in response to temperature rise in the room being controlled, a spring for biasing said lever against the action of said-:temperature responsive device,` an adjust ingmembercfor'adjusting the bias of 'said spring,

a temperature calibrated scaleadj'acent' said adl2 justing member said adjusting member indicating on said scale the adjustment of'v said" spring bias, stop members for limiting the angle of movement of said lever to a movement', corresponding to a two degree temperature response oi said temperature control device, a member rigidly secured to and movable with said lever and carrying a first movable contact, an arm carried by said member and having a` second movable contact secured thereto for engaging and: disengaging said rst movable contact', a cam having a surface engaged by said arm, a motor connected to and driving saidv cam,l the connection from said. motor to said cam being constructed to drive said cam4 at a speedlofjonequarter R. P; M said, cam during one portion of its rotation bein-g in engagement with said arm and moving said second'L movable contact toward disengagement'with said first contact and said rst contact being movedv toward'disengagementY with said second contact by expansion of said temperature'responsive deviceA in accordance with the settingV of said biasing spring to effect engagement between said contacts forvariable periods of time during each complete revolution of said cam.

DONALD I. BOHN.

REFERENCES CITED The following references are of record inthe l'e ofthis patent:

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